1. Field of the Invention
The present invention relates to a thermosensitive color printing method and a thermosensitive color printer for printing a full-color image on thermosensitive color recording paper in a frame sequential fashion. More particularly, the present invention relates to a thermosensitive color printing method and a thermosensitive color printer which reduce color failures and smooth the surface of the thermosensitive color recording paper after having an image recorded thereon.
2. Background Arts
In a thermosensitive color printer, thermosensitive color recording paper, hereinafter called simply the recording paper, is directly headed by a thermal head that is pressed onto the recording paper while the recording paper is transported. As the recording paper is heated, color dots are developed on the recording paper.
As shown in FIG. 11, the recording paper 10 has a thermosensitive cyan coloring layer 12, a thermosensitive magenta coloring layer 13, and a thermosensitive yellow coloring layer 14 formed atop another on one side of a base material 11. A transparent protective layer 15 is formed atop the thermosensitive coloring layers 12 to 14, for protecting the coloring layer 12 to 14 from scratches or stains. The sequence of forming these three coloring layers 12 to 14 is not limited to that shown in the drawings, and the three coloring layers 12 to 14 have different heat-sensitivities from each other that decrease with the depth or distance of the respective layers from an obverse surface 10a of the recording layer 10. Intermediate layers 16 and 17 are formed between these three coloring layers 12 to 14, for adjusting the heat-sensitivities of the respective coloring layers 12 to 14. A back protective layer 18 is formed on the opposite side of the base material 11 from the protective layer 15.
In the recording paper 10 shown in FIG. 11, the cyan coloring layer 12 has the lowest heat-sensitivity and the yellow coloring layer 14 has the highest heat-sensitivity. Accordingly, as shown in FIG. 12, the yellow coloring layer 14 needs the smallest heat energy to develop yellow color, whereas the cyan coloring layer 12 needs the largest heat energy to develop cyan color. Because of the difference in heat-sensitivity between the three coloring layers 12 to 14, it is possible to record three color frames sequentially from the highest sensitive coloring layer to the lower sensitive coloring layer by applying increasing amounts of heat energy to the recording paper 10 from one color after another.
To stop the coloring layer from being developed unnecessarily by the heat energy applied for recording the next color frame, the coloring layer having a color frame recorded thereon is fixed by electromagnetic rays of a specific range before the next color frame is recorded. In the recording paper 10, the magenta coloring layer 13 has an absorption spectrum whose peak wavelength is at about 365 nm, and loses coloring ability when it is exposed to ultraviolet rays of this wavelength range. On the other hand, the yellow coloring layer 14 has an absorption spectrum whose peak wavelength is at about 420 nm, and loses coloring ability when it is exposed to violet visible light of this wavelength range. So the violet visible light of 420 nm is projected onto the recording paper 10 after the yellow frame is recorded, before the magenta frame is recorded. After recording the magenta frame, the ultraviolet rays of 365 nm is projected onto the recording paper 10 to fix the magenta coloring layer 13.
The thermosensitive printer uses a platen driven type paper transport device or a capstan driven type paper transport device. The platen driven type feeds the recording paper by rotating a platen roller, whereas the capstan driven type feeds the recording paper by rotating a capstan roller that is provided besides the platen roller.
The capstan roller must be made of a metal with high accuracy, so it is expensive. In addition, it needs a rubber nip roller as a counterpart. Since the platen roller itself is expensive, the capstan driven type costs more compared to the platen driven type. Moreover, because the thermal head must be pressed onto the recording paper with a certain pressure to achieve a sufficient image quality, the load for feeding the recording paper is relatively large, so color failures are easy to occur when the feeding power of the capstan roller is weak.
In the platen driven type, on the other hand, the recording paper is turned a certain angle around the platen roller to improve the feeding power of the platen roller, so a large margin is necessary between a leading end of the recording paper and an image recording area.
The protective layer 15 is made from a transparent plastic resin material. The protective layer 15 also contains various additives to prevent blocking or the like. Because of the additives, the surface of the protective layer 15 is rough. As well-known in the art, the plastic resin material starts to be softened above a glass transit point or temperature of the main component of the resin material. The glass transit point varies between different resin materials. For example, a thermosensitive color recording paper that has been marketed under a trade name Thermo Auto Chrome Pater A-20 by Fuji Photo Film Co., Ltd., uses PVA (poly-vinyl-alcohol) as the main component of the protective layer, and the glass transit temperature of PVA is about 70.degree. C.
Because different amounts of heat energy are applied to the recording paper for developing different colors, and the protective layer is softened in different degrees by the different heat energies, friction factor between the thermal head and the protective layer of the recording paper varies depending upon which color frame is being recorded. Therefore, color failure is likely to occur in the thermosensitive color printer unless the difference in friction factor between colors is not taken into consideration.
The thermal head has an array of glaze layers formed on an alumina substrate, and a heating element is located at a peak of a semi-cylindrical glaze layer that extends crosswise to the paper transporting direction. Since the heat energy applied to the recording paper for developing colors is so high, the temperature of the protective layer of the heated portion can be still above its glass transit point even after it is moved off the glaze layer. In that case, additives contained in the protective layer, such as an anti-blocking agent, emerge to the obverse surface of the recording paper, providing irregular fine roughness on the obverse surface. The roughness lessens glossiness of the obverse surface, and coarsens the printed image.
To restore the glossiness of the recording paper after having an image recorded thereon, according to a conventional smoothing process, a flat smooth sheet is laid over the recording paper, and the recording paper is squeezed together with the flat smooth sheet through a pair of heating rollers, thereby to hot-press the recording paper.
However, this conventional smoothing process needs a specific smoothing apparatus is necessary in addition to the printer, and laying the flat smooth sheet over the recording paper is labor-consuming. Moreover, because the heating rollers contain heaters therein, it is hard to take fine control of heat energy applied from the heat rollers to the recording paper, although the optimum range of heat energy for obtaining the highest glossiness is narrow and limited.
It may be possible to provide a thermosensitive printer with a second thermal head for smoothing in addition to a thermal head for recording, so as to heat the recording paper uniformly by the second thermal head after three color frames are sequentially recorded by the first thermal head. It is also possible to use the same thermal head for recording and smoothing, for example by uniformly heating the recording paper, after having three color frames recorded thereon, by the same thermal head while transporting the recording paper in the same direction as it is transported during recording.
However, optimum contacting conditions of the heating elements with the protective layer for smoothing are different from those optimum for recording. Therefore, it has been difficult to achieve both adequate coloring quality and highest glossiness by using the same thermal head for recording and smoothing.